Coherent optical multichannel correlator

ABSTRACT

A coherent optical multichannel correlator having a plurality of channels of coherent light beams for permeating an object to be identified, a Fourier transmission lens receiving the light permeating the object, a number of filter holograms disposed on a focal plane of the transformation lens and one or more photo detectors for receiving the output of the filter holograms characterized by providing a matrix of controllable optical switches between the Fourier transformation lens and the photo detectors for sequentially blocking and passing selected channels to reduce background noise from cross correlation of adjacent channels. The matrix of optical switches can be an arrangement of mechanically actuated light shutters or a matrix of electrooptical polarization switches.

C United States Patent 1 1 1111 3,776,616

Douklias Dec. 4, 1973 COHERENT OPTICAL MULTICHANNEL 3,703,640 11 1972Broussaud et a1. 350/162 x CORRELATOR 75 Inventor: Nikolaos Douklias,Munich, P'imary Examinerr-william Lindquis Germany AttorneyBenjamin H.Sherman et a1.

[73] Assignee: Siemens Aktiengesellschaft, Berlin and Munich, Germany[57] ABSTRACT [22] Filed: Oct 10, 1972 A coherent optical multichannelcorrelator having a plurality of channels of coherent light beams forper- [21'] PP N05 295,998 meating an object to be identified, a Fouriertransmission lens receiving the light permeating the object, a [30]Foreign Application Priority Data number of filter holograms disposed ona focal plane Nov 1971 German P 21 578 75 6 of the transformation lensand one or more photo dey tectors for receiving the output of the filterholograms haracterized b rovidin a matrix of controllable 2 .1 C y p g 55 350/162 optical switches between the Fourier transformation [58]Fie'ld SF 356/71 lens and the photo detectors for sequentially blockingand passing selected channels to reduce background 56] References Citednoise from cross correlation of adjacent channels. The

matrix of optical switches can be an arrangement of I UNITED STATESPATENTS mechanically actuated light shutters or a matrix of 3,597,0698/1971 l-leinonen, Jr. 350/162 X electr ptical polarization switches.3,644,019 2/1972 Bestenreiner et a1 350/162 3,671,106 6/1972 Groh350/162 7 Claims, 1 Drawing Figure llIJ LLl 1 COHERENT OPTICALMULTICIIANNEL CORRELATOR BACKGROUND OF THE DISCLOSURE 1. Field of theInvention The present invention relates to a coherent opticalmultichannel correlator utilizing a plurality of coherent light beams aschannels for illuminating an object which is to be examined, a Fouriertransformation lens, a number of filter holograms and several photodetectors.

2. Prior Art For the automatic recognition of a two-dimensional symbolor character such as an alpha-numeric sign, it has been suggested to usecoherent optical correlators which have a source of coherent light, suchas a laser, means for supporting the symbols on one focal plane of aFourier transformation lens and a spatial filter, such as filterhologram located in the opposite focal plane. The coherent light, whichpermeates the symbol and is focused by the transformation lens, producesan object wave, and, when this object wave coincides with the objectwave of the hologram filter, a reference wave or beam is reconstructedto provide a useful recognition signal. For a recognition, of atwo-dimensional sign, a certain tolerance range is required in thesignal input plane for the position coordinance of the sign to obtainthe recognition signal which is an auto-correlation signal. Theholographic matched filters provide an increase of tolerances fortranslational variations for the position coordinator of the object tobe identified and the holographic correlation is translationalinvariant.

However, rotational variations in the coordination of theobjects'requires a recognition system utilizing a multichannelcorrelator.

The term translation invariant means the auto correlation signal incursindependent from the shifting of the signs in a holographic correlator.However, it does not mean that the signal is produced in the detectorplane in the same place and the position for the maximum of theauto-correlation signal changes with the changes in the position of thesign. The size of the detector element thus depends not only on thediameter of the auto-corre lation signal but also on possible shiftingsof the position of the signal due to shifting in the position of thesign in the input plane.

With a parallel evaluation of all channels of a multichannel correlator,all combinations of each sign with each filter will be produced in thedetector plane as correlation signal. An unfavorable signal tointerference ratio or signal to noise ratio is produced when the amountsof adjacent cross-correlation terms which occur during parallelevaluation are superimposed on the auto-correlation maximum. In order toobtain a good signal separation from the interference signal or noisesignal while simultaneously providing filter readiness of all filters,an expensive detector matrix which is adapted to a multiple ofcorrelation fields is required.

A coherent optical multichannel correlator with a high signal to noiseratio is discussed in the copending United States patent application,Ser. No. 252,022, which was filedon May 10, 1972, and the correlatorutilized a modulator which enabled a sequential modulation of thecarrier frequency. This multichannel correlator has certain drawbacks inthat the modulator included mechanically moving parts which have acertain inertia. Furthermore, an exact positioning of the mechanicalmoving parts for eachof the individual channels requires an expensivealignment process. inexact SUMMARY OF THE INVENTION The presentinvention is directed to providing a co herent optical multichannelcorrelator which allows a fast and precise sequential control of thechannels without the losing of the advantages of the multichannelcorrelators. The correlator comprises means for providing a plurality ofbeams of coherent light for permeating an object to be identified, aFourier transformation lens for receiving the beams of light afterpermeating the object being identified, a plurality of filter hologramslocated on the focal point of the lens, optical sensing means forsensing reconstructed reference waves from the filter holograms and acontrollable means for selectively blocking and passing the beams ofcoherent light which controllable means is disposed between the lens andthe optical sensing means. The means for providing the multiple beamspreferably comprises a single source of coherent light and an opticaldiffraction grating which separates the light from the single sourceinto multiple beams. The controllable means for blocking and passing ispreferably an optically switching arrangement and comprising a matrix ofoptical switches with each switch having a dimension for thecorresponding dimensions of the channel and being associated with thechannel of coherent radiation. In one embodiment of the invention, theoptical switches are mechanically actuated shutters. To provide a highspeed in sequentially determining the output of each of the hologramfilters, the switching matrix is preferably a plurality ofelectro-optical polarization switches and the means providing aplurality of coherent light beams provides polarization of the beams ofthe coherent light. The use of electro-optical switches has anadditional advantage of no moving parts.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a schematic illustrationof an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The principles of the presentinvention are particularly useful for a multichannel correlatorillustrated in the FIGURE. The multichannel correlator utilizes amonochromatic coherent light beam 1, such as produced by a laser, whichis passed through a diffraction grating 2 which produces a plurality ofplane, coherent beams or channels of light which permeates an object 3which is to be examined. A Fourier. transformation element such as lens4 is arranged next to the object 3 so that the object 3 is in a firstfocal plane thereof. A plurality of hologram filters are arranged as amatrix 5 in the second focal plane of the lens 4 to receive the lighttherefrom. Each of the hologram filters coacts with one of the multiplebeams to provide a correlation channel and each hologram filter isconstructed in such a way that the reference wave which is producedduring auto-correlation is focused on an optical sensing means 7 whichis illustrated as laterally arranged or offset thereto. As illustrated,the optical sensing means may include a plurality of photo-detectorswith each detector being positioned to receive a reference wave from anindividual filter hologram in the matrix 5 of filter hlograms.

To prevent an unfavorable signal to interference ratio or signal tonoise ratio due to the superimposition of cross-correlation terms ofadjacent filters on the auto-correlation maximum, a controllable means 6for sequentially blocking and passing the selected channels is providedbetween the means for sensing 7 and the lens 4. As illustrated, themeans 6 for sequentially blocking and passing is a matrix of opticalswitches with each of the switches being of a dimension corresponding tothe dimension of a channel from a diffraction grating 2 and associatedtherewith. Thus the matrix 6 can be arranged either ahead or behind thematrix 5 of hologram filters. As illustrated, the matrix 6 is located ina position to block or pass selected reconstructed reference waves fromthe holograms of matrix 5. The switches of the matrix 6 are sequentiallyactuated by means to enable the sequential determination of the outputof the filter holograms in the matrix 5. By utilizing electro-opticalpolarization switches which are controlled by electronic means, a fastsequential determination of the outputs of all of the filter hologramscan be obtained. When electro-optical polarization switches areutilized, the light beams permeating the object to be identified must bepolarized which is accomplished by providing the polarization meanseither with the laser of the source or in the beam path. therefrom. Ifvery high speeds are not demanded, the role or function of the opticalswitches can be performed by correspondingly dimensioned opticalshutters such as mechanically actuated rotating apertures. In eitherembodiment of the switches, the means controlling the operation of theswitch is synchronized with the means evaluating the output of thesensing means 7 in a conventional manner.

The evaluation by the sensing means 7 during the parallel opticalmaximum processing of several signs is generally effected sequentially.However, a certain time is required for completing an evaluation due tothe switching time for the optical switch arrangements. Since no amountof cross-correlation factors can become recognizable in an interferingmanner at the place of the auto-correlation maximum, the degree ofefficiency of the entire arrangement during the sequential signalprocessing is essentially higher than with the parallel signalprocessing in spite of the utilization of a simple detector matrix forthe sensing means.

When the matrix 6 utilizes electro-optical switches, all mechanicalmoving parts will be eliminated. Furthermore, the switching time of theindividual switches can be lowered in such a way that they do not haveany limiting effect on an electrical evaluation means or system, whichreceives the output of the sensing means, during the evaluation of therecognition signals.

Since the available light intensity from the single source issimultaneously fanned out or spread over several channels, the intensityof light available in each of the individual channels is anticipated tobe smaller than the intensity of the light in a single channeloperation. To compensate for this, it is suggested that either thephoto-detectors of the matrix 7 have a high sensitivity or the lightsource be provided by a laser having a higher output intensity.

Although various minor modifications might be suggest by those versed inthe art, I wish to incorporate within the scope of the patent grantedhereon all such variations and modifications that reasonably come withinmy contribution to the art.

I claim:

1. A coherent optical multichannel correlator for identifying objectscomprising: means for providing a plurality of beams of coherent lightas channels for permeating an object to be identified; a Fouriertransfor mation lens for receiving the beams of light after permeatingthe object to be identified, a plurality of filter holograms located onthe focal plane of the lens, optical sensing means arranged forreceiving reconstructed reference waves from the filter holograms, andcontrollable means for sequentially blocking and passing the beams ofcoherent light, said controllable means being disposed between the lensand the optical sensing means so that the optical sensing meanssequentially senses the reconstructed waves of selected hologramfilters.

2. A coherent optical multichannel correlator according to claim 1,wherein the controllable means for sequentially blocking and passingcomprises a matrix of optical switch elements with each switch elementhaving a dimension for a channel of the coherent light, and said switchelements being arranged with a switch element associated with each ofthe channels.

3. A coherent optical multichannel correlator ac cording to claim 1,wherein the controllable means for sequentially blocking and passingcomprises a matrix of optical shutters which are mechanically operatedwith an optical shutter associated with each of the channels.

4. A coherent optical multichannel correlator according to claim 3,wherein the matrix of optical shutters is disposed between the hologramfilters and the sensing means so that the reconstruction wave from eachhologram filter is sequentially passed to the sensing means.

5. A coherent optical multichannel correlator according to claim 1,wherein the means for providing a plurality of light beams includesmeans for polarizing each of the beams and wherein the controllablemeans for sequentially blocking and passing the light includes a matrixof individually controllable electro-optical polarization switches witha switch associated with each of the channels.

6. A coherent optical multichannel correlator according to claim 5,wherein the matrix of electrooptical polarization switches is disposedbetween the hologram filters and the sensing means so that thereconstructed wave from each of the filter holograms is sequentiallypassed to the sensing means.

7. A coherent optical multichannel correlator according to claim 1,wherein the means for providing a plurality of beams with coherent lightcomprises a source producing a beam of coherent light and an opticaldiffraction grating receiving the beam of light and diffracting it intoa plurality of coherent beams.

1. A coherent optical multichannel correlator for identifying objectscomprising: means for providing a plurality of beams of coherent lightas channels for permeating an object to be identified; a Fouriertransformation lens for receiving the beams of light after permeatingtHe object to be identified, a plurality of filter holograms located onthe focal plane of the lens, optical sensing means arranged forreceiving reconstructed reference waves from the filter holograms, andcontrollable means for sequentially blocking and passing the beams ofcoherent light, said controllable means being disposed between the lensand the optical sensing means so that the optical sensing meanssequentially senses the reconstructed waves of selected hologramfilters.
 2. A coherent optical multichannel correlator according toclaim 1, wherein the controllable means for sequentially blocking andpassing comprises a matrix of optical switch elements with each switchelement having a dimension for a channel of the coherent light, and saidswitch elements being arranged with a switch element associated witheach of the channels.
 3. A coherent optical multichannel correlatoraccording to claim 1, wherein the controllable means for sequentiallyblocking and passing comprises a matrix of optical shutters which aremechanically operated with an optical shutter associated with each ofthe channels.
 4. A coherent optical multichannel correlator according toclaim 3, wherein the matrix of optical shutters is disposed between thehologram filters and the sensing means so that the reconstruction wavefrom each hologram filter is sequentially passed to the sensing means.5. A coherent optical multichannel correlator according to claim 1,wherein the means for providing a plurality of light beams includesmeans for polarizing each of the beams and wherein the controllablemeans for sequentially blocking and passing the light includes a matrixof individually controllable electro-optical polarization switches witha switch associated with each of the channels.
 6. A coherent opticalmultichannel correlator according to claim 5, wherein the matrix ofelectro-optical polarization switches is disposed between the hologramfilters and the sensing means so that the reconstructed wave from eachof the filter holograms is sequentially passed to the sensing means. 7.A coherent optical multichannel correlator according to claim 1, whereinthe means for providing a plurality of beams with coherent lightcomprises a source producing a beam of coherent light and an opticaldiffraction grating receiving the beam of light and diffracting it intoa plurality of coherent beams.